The long term objectives of this work are to contribute to the elucidation of the structural organization and development of the vertebrate visual system and to clarify the changes that occur in it when it is deprived of its normal visual input. The studies we propose bear directly on the important issue of how the visual system is assembled during embryonic development and on the problems that may result from a variety of developmental abnormalities and from sensory deprivation. In the forthcoming grant period we propose specifically to examine the following: (i) The normal pattern of development of the avian optic tectum and its innervation by the retina, using 3H-thymidine autoradiographic and immunocytochemical techniques, monoclonal antibodies raised against tectal antigens, and a number of experimental manipulations including 180 degree rotation of the alar plate and/or midbrain, and the rerouting of the retinal input to the tectum. (ii) The capacity of the regenerated chick optic tectum for reorganization and morphological plasticity following early unilateral eye removal. (iii) The effects of sensory overloading on the chick tectum and the nucleum of origin of centrifugal fibers to the retina (the so-called isthmo optic nucleus-ION) induced by transplanting supernumerary eyes and producing "compound" eyes. (iv) The factors involved in the normal elimination of aberrant and exuberant retinal projections in chicks and hamsters, by early manipulations of the retina, optic nerve and optic tectum. (v) The factors responsible for the "guidance" of centrifugal axons to the chick retina and the fate of ectopic or misplaced centrifugal neurons. (vi) The regulation of naturally-occurring cell death in the retina and ION and especially the role of activity in the maintenance of neurons in these structures.